Hydraulic pumps and motors may generally be divided in two main categories, fixed displacement devices and variable displacement devices. The advantage with a variable displacement device is quite obvious in that it is possible to vary the volume capacity of the device in a rather easy way. However, since there is a possibility to vary the displacement volume there is also a desire to indicate and control the displacement volume.
In U.S. Pat. No. 6,848,888 is disclosed a magnetic sensor used to indicate the position, or angle, of a swash plate relatively its housing in order to decide the displacement volume. U.S. Pat. No. 7,275,474 describes a system using markings on a swash plate, yoke or the like movable part of the hydraulic device which may be read optically or electronically by a sensor fixedly located on the housing in order to decide the absolute position of the movable part. U.S. Pat. No. 4,822,252 describes the use of a magnetic arrangement in order to detect the degree of inclination of a wobble plate of a variable capacity compressor. Still further arrangement for detection of a swash plate angle or yoke position is disclosed in for example U.S. Pat. No. 5,881,629; U.S. Pat. No. 5,135,031 or U.S. Pat. No. 5,073,091.
Even though the above cited documents provide different solutions for detecting the displacement of a variable displacement hydraulic devices there is still a desire to improve such a detecting system and provide a more robust and safe system for sensing the position of the moveable part, e.g. a yoke, in order to better control the hydraulic system